Split plenum arrangement for an apparatus for coating tablets

ABSTRACT

A fluidized bed apparatus for applying a coating liquid onto the surface of particles includes a vertically disposed cylindrical product container having a peripheral wall, at least one cylindrical partition defining a centrally located up bed region and a peripherally located down bed region. The product container further includes an upper end connected to an expansion chamber and a lower end including an orifice plate having a plurality of openings for passage of fluidized air or gas. A nozzle is centrally located through the orifice plate and is adapted to generate a spray of coating liquid upwardly into the up bed. Particles located within the product container circulate upwardly through the partition and the coating liquid spray, between the up bed and the down bed. A plurality of inwardly directed discharge jets are positioned along the periphery of the product container and generally adjacent to the orifice plate. A source of pressurized air or gas is connected to the plurality of discharge jets and provide a radially inwardly directed flow of air or gas through the particles of the down bed so that the particles located adjacent to the peripheral wall of the product container are displaced radially inwardly along the orifice plate towards the nozzle and the up bed. A partition having an outer wall surface and an outwardly directed flared lower end defining a lower rim. The partition further includes an inwardly and upwardly directed ramp surface extending between the lower rim and the up bed. The improved partition encourages particles located within the down bed adjacent to the outer wall surface to move outwardly away from the up bed until reaching the lower rim, at which point the particles are atraumatically and gradually directed inwardly and upwardly along the inwardly and upwardly directed ramp surface into the up bed to continue the coating process. A nozzle ramp is provided around the nozzle. The nozzle ramp is generally cusp shaped and includes nozzle-ramp surface. The nozzle ramp is centrally positioned around the nozzle and is directed upwardly towards the partition so that the nozzle-ramp surface directs particles moving generally horizontally across the orifice plate from the down bed upwardly into the partition and the up bed. The nozzle ramp may further include air passages for passing fluidized air upwardly to the nozzle-ramp surface. A central discharge assembly is provided at the lower end of the product containers for emptying particles from the machine. Air is preferably discharged from the manifold during the emptying process to help direct particles towards the discharge assembly.

[0001] This patent application claims priority of U.S. ProvisionalPatent Application, serial No.: 60/166,799, filed Nov. 22, 1999, whichis incorporated herewith in its entirety.

BACKGROUND OF THE INVENTION

[0002] A. Field of the Invention

[0003] The present invention generally relates to coating machines and,in particular, to fluid-bed coating machines used for the coating oftablets.

[0004] B. Description of the Prior Art

[0005] Tablets are formed by pressing pharmaceutically active drugs,filler and binding agents together. Once formed, it may be necessary, ordesirable to provide the tablet with a coating which will:

[0006] 1. prevent any portion of the drug from being released, such asin the form of dust;

[0007] 2. mask any unpleasant odor or taste of the active drug, or anyfiller or binder used;

[0008] 3. facilitate swallowing by providing a smoother and lessabsorbent outer layer;

[0009] 4. protect the contents of the tablet from pre-mature digestionby providing a coating which is resistant to gastric fluids;

[0010] 5. control the rate of absorption of the drug by the smallintestine; and

[0011] 6. improve the appearance of the tablet and provide a printablesurface.

[0012] The tablets are generally coated using machines which spray acoating material, such as hydroxypropylmethylcellulose onto the surfacesof the tablets while the tablets are in motion within a productcontainer. Two common types of machines tumble tablets within ahorizontally rotatable drum during the spraying process, while anothertype of tablet coating machine uses a vertical flow of air to circulatetablets past a vertically disposed spray nozzle. The prior art coatingmachines are described below:

[0013] 1. Dragee Kettle

[0014] For most applications, the exact thickness of the coated layer isnot critical and many different types of coating machines may be used toapply a crude, yet effective coating to the tablet. An older oncepopular type of coating machine is called a dragee kettle and examplesof these machines are disclosed in U.S. Pat. Nos. 3,831,262 and5,334,244. This machine includes a large drum-like vessel which istypically rotated about a horizontal axis. The vessel includes a coatingchamber which is partially filled with tablets to be coated so that asthe vessel rotates, the tablets roll and tumble along the inside wall ofthe coating chamber. During this tumbling motion, coating materials inthe form of aqueous or organic suspensions of liquids are sprayedthrough nozzles and into contact with the rolling tablets within thecoating chamber. During the coating process, a current oftemperature-controlled air circulates in the coating chamber of thedragee kettle, which helps evaporate the suspension agent of the coatingmaterial so that the coating material effectively dries and adheres tothe tablets.

[0015] One problem with the dragee kettle coating machine is thattypically the tablets are not the only surfaces coated within thecoating chamber. Even when a carefully controlled spraying schedule isfollowed (such as spraying at very short intervals while the drageekettle rotates), much of the sprayed coating material still ends up onthe inside wall of the coating chamber, as well as throughout theevaporation/venting ducting. This over-spraying creates numerouscontamination and cleaning problems, and further increases the cost ofthe coating since much of the coating material is lost during thecoating process.

[0016] The above-described dragee kettle type coating machine is limitedto coating tablets which do not require much precision in the thicknessof the coated layer because the thickness of the coating of the tabletswill vary in the same batch. This process may be used to coat manydifferent types of pharmaceuticals, vitamins, and even candy, as long asuniform coating distribution and thickness are not required.

[0017] 2. The next generation of tablet coating machines after thedragee kettle is called a perforated pan tablet coating machine. Thismachine has improved the tablet coating process and is the most commontype of tablet coating machine in use today. The perforated pan machineincludes a rotatable perforated drum which rotates about a horizontalaxis within a housing, and further includes a plurality of nozzlespositioned within the drum. The nozzles create a spray of coatingmaterial within the drum so that any tablets located within the drumwill tumble about into and out of the spray pattern and, over a periodof time, will accumulate a coating on their surface. An importantimprovement of the perforated pan coating machine over the dragee kettleis that the perforated pan machine allows air directed through thehousing (using appropriate ducting) to pass through the perorated drumand quickly reach the tablets tumbling therein. The perforations of thedrum effectively expose the tumbling tablets to the current of air,resulting in more uniform distribution of drying air for each tablet.The drum further includes solid baffles which are used to enhance mixingof the tablet bed in an effort to improve the distribution of thematerial being sprayed onto the tablets.

[0018] 3. Fluidized Bed Coating Machines

[0019] Another type of particle-coating apparatus is called a fluidizedbed coating machine (also known as a Wurster machine, after inventorDale Wurster). Several examples of the Wurster coating machine aredisclosed in U.S. Pat. Nos. 3,196,827,3,110,626, 3,880,116, 4,330,502,4,535,006 and 5,236,503.

[0020] The Wurster coating machine is typically used to layer, coat orencapsulate lightweight powders, particles, granules or pellets of solidmaterials, including pharmaceutical drugs. Often, coatings are appliedto modify the release of the substrate (protective barrier, tastemasking, enteric coating, delayed release or sustained release). Apredetermined quantity of these coated particles are usually packagedwithin an edible gelatin capsule or compressed into a tablet. Thedistribution uniformity of the applied substance may not be criticalbecause the capsule or tablet contains multiple units and the averagecoating thickness of all of the pellets within the capsule dictate theaverage release properties and performance of the overall dosage form.

[0021] As described below, the Wurster machine generates an upwardstream of air or other gases such as nitrogen to circulate a substrate(particles, pellets, powders, etc.) through a vertical spray of coatingliquid within a product container. As the substrate cycles through thespray, a minute amount of coating material is deposited on its surface.The number of cycles the substrate completes determines the thickness ofthe final coating layer.

[0022] The conventional Wurster machine works well when the particlesare fine and lightweight (such as grains of powder). However, due toflow-related problems inherent in the design, the conventional Wurstermachine fails to provide a uniform distribution of coating on heaviertablets because the heavier tablets do not uniformly cycle through themachine. The Wurster-coated tablets cannot be used for applicationswhich require uniform, predictable and consistent distribution coatingson all tablets within a particular batch.

[0023] Certain types of pharmaceutical controlled-released tabletsrequire high-precision coatings because the thickness of the coatinggoverns the time of release and the release rate of the activeingredient of the tablet and thereby directly influences theeffectiveness of the medication. The conventional Wurster machine isincapable of providing a high-precision coating on tablets, in part dueto the following flow-related problems, each of which adversely effectsthe precision of the coating of each tablet or particle in the batch.

[0024] The conventional Wurster machine also creates undesirableturbulence and introduces high shear forces to the substrate as itcycles through the machine. The fine and lightweight substratestypically used with conventional Wurster machines are not adverselyaffected by the violent traumatic forces they must endure during eachcycle. However, when a conventional Wurster machine is used to coatheavier tablets, the high shear forces generated during each coatingcycle are capable of damaging the tablets and the resulting attritionrate of the tablets is unacceptable.

[0025] The heavier tablets are also more difficult to introduce into thehigh velocity airstream of the Wurster machine, usually causing some ofthe tablets to accelerate directly into hard structures within themachine, such as a nozzle assembly. The impact can easily shatter orotherwise damage the tablets.

[0026] Once a substrate is processed using the Wurster machine, thesubstrate must be removed from the product container. This isconventionally accomplished through a pivotal bottom door which, whenopened, allows the coated substrate to simply fall by gravity into anawaiting and suitable container. Although this emptying process iseffective, the process exposes both the substrate and the interior ofthe product container to the environment. Not only does this exposureintroduce undesirable contamination to the product container, it alsosubjects the operators of the machine unnecessarily to potentiallyhazardous materials. To this end, it would be beneficial to remove thecoated substrate from the coating machine using a more controlled andpredictable process without undue complexity and without affecting themachine's operation.

[0027] Another problem with Wurster machines is that they are relativelydifficult to clean. The cleaning procedure typically requires theopening of the lower end of the product container and the application ofan appropriate cleaning fluid. Some coating machines have spray nozzleswithin the coating machine to initially wash out any residual materialdeposited along the interior surfaces of the machine after the coatingprocess. The cleaning fluid from these nozzles washes the interiorsurfaces of the machine and typically drains through the open lower end.Sometimes, however, the material being processed within the productcontainer comprises a drug or other material which may be hazardous ifaccidentally inhaled, swallowed or even touched by personnel assigned tooperate and clean the coating machine. It would therefore be beneficialto ensure that a maximum amount of this potentially hazardous residue iswashed from the expansion chamber and the product container whileisolating the contaminated waste from the surrounding environment.(i.e., without having to opening the machine).

[0028] Objects of the Invention

[0029] It is an object of the present invention to provide afluidized-bed type coating machine which overcomes the deficiencies ofthe prior art.

[0030] It is another object of the present invention to provide aWurster-type coating machine which encourages even and predictable flowof tablets located in the down-bed.

[0031] It is another object of the present invention to provide aWurster-type particle-coating machine which encourages tablets to flowradially inwardly along a distribution plate between the down-bed and anup-bed.

[0032] It is another object of the invention to provide a Wurster-typeparticle-coating machine which includes a central nozzle assemblylocated at the distribution plate for discharging a spray of coatingliquid and which further includes structure to redirect tablets from thedown-bed to the up-bed without impacting the central nozzle assembly.

[0033] It is another object of the invention to provide a Wurster-typeparticle-coating machine which is particularly suited to accurately coatheavier particles, such as tablets.

[0034] It is another object of the present invention to provide aWurster-type particle-coating machine which includes a partition whichis shaped to provide an atraumatic transition of the tablets moving fromthe down-bed into the up-bed.

[0035] It is another object of the present invention to provide aWurster-type particle-coating machine which cycles the tablets withinthe machine between the down-bed and the up-bed in a smooth, efficient,and consistent manner so that the resulting coating distribution of eachtablet is consistent and predictable and tablet-attrition is minimized.

[0036] It is another object of the present invention to provide aWurster-type particle-coating machine which permits discharge of thesubstrate (particles or tablets) through an opening at the center of theorifice plate at the base of the insert when multiple partitions areused.

[0037] It is another object of the present invention to provide aWurster-type particle-coating machine which is easy to operate duringthe coating process and facilitates cleaning without disassembly of theWurster insert.

SUMMARY OF THE INVENTION

[0038] The foregoing objects of the invention are met through variousimprovements to a Wurster-type fluidized bed apparatus for applying acoating liquid onto the surface of particles. The coating liquid isgenerally comprised of substances in a solution, suspension ordispersion in water or organic solvent (in some cases a molten liquidmay be used). The apparatus includes a vertically disposed cylindricalor slightly conical product container having a peripheral wall, at leastone cylindrical partition defining a centrally located up bed region anda peripherally located down bed region. The product container furtherincludes an upper end connected to an expansion chamber and a lower endincluding an orifice plate having a plurality of openings for passage offluidized air. A nozzle is centrally located through the orifice plateand is adapted to generate a spray of coating liquid upwardly into theup bed. Particles located within the product container circulateupwardly through the partition and the coating liquid spray, between theup bed and the down bed.

[0039] A first feature of the invention comprises a plurality ofinwardly directed discharge jets positioned along the periphery of thecylindrical product container and generally adjacent to the orificeplate. A source of compressed air or other inert oases such as nitrogenis connected to the plurality of discharge jets. The flow of compressedair through the jets provides a radially inwardly directed airstreamthrough the particles of the down bed so that the particles locatedadjacent to the peripheral wall of the product container are displacedradially inwardly along the orifice plate towards the nozzle and the upbed.

[0040] A second feature of the invention comprises a partition for usein a Wurster-type coating machine. This partition includes an outer wallsurface and an outwardly directed, flared lower end defining a lowerrim. The partition further includes an inwardly and upwardly directedramp surface extending between the lower rim and the up bed. Thepartition encourages particles located within the down bed adjacent tothe outer wall surface to move outwardly away from the up bed untilreaching the lower rim, at which point the particles are atraumaticallyand gradually directed inwardly and upwardly along the inwardly andupwardly directed ramp surface into the up bed to continue the coatingprocess.

[0041] A third feature of the invention comprises a nozzle ramp which isplaced around the nozzle. The nozzle ramp is generally cusp shaped andincludes a nozzle-ramp surface. The nozzle ramp is centrally positionedaround the nozzle and is directed upwardly towards the partition so thatthe nozzle-ramp surface directs particles moving generally horizontallyacross the orifice plate from the down bed upwardly into the partitionand the up-bed. The nozzle ramp may further include air passages forpassing fluidized air upwardly to the nozzle-ramp surface. The air flowthrough these perforations provides a cushion of air at the surface ofthe nozzle ramp which minimizes the impact of tablets against thisstructure. The purpose of the perforated nozzle ramp is toatraumatically guide the horizontally flowing tablets vertically andupwardly into the up bed inside the partition.

BRIEF DESCRIPTION OF THE DRAWINGS

[0042]FIG. 1 is a sectional side view schematic (labeled PRIOR ART) of aprior art Wurster-type bottom spray particle-coating machine showing aproduct container, a partition, a nozzle, a down-bed, and a up-bed, anozzle sleeve, and a plurality of tablets or pills (represented byspheres) being coated as they circulate through the machine.

[0043]FIG. 2 is an enlarged sectional view (labeled PRIOR ART) of aportion of the prior art Wurster-type coating machine of FIG. 1 (shownwithout the numerous particles, for clarity) including particle-flowarrows representing the flow of particles during their transition fromthe down-bed into the up-bed air-stream, and further including a dashedline representing a “dead-zone” within the product container;

[0044]FIG. 3 is a sectional side view schematic of an improvedWurster-type bottom spray particle-coating machine showing a productcontainer, a partition having a peripheral skirt, a nozzle, a down-bed,an up-bed, a nozzle sleeve, a truncated cusp-shaped nozzle-ramp, and aplurality of tablets or pills (represented by spheres) being coated asthey circulate through the machine, according to the present invention;

[0045]FIG. 4 is an enlarged sectional view of a portion of the improvedWurster-type coating machine of FIG. 3 (shown with only selectedparticles, for clarity) showing details of the peripheral skirt, thenozzle-ramp, the nozzle sleeve, and the peripheral air-injection systemand further including particle-flow arrows representing the flow of theselected particles during their transition from the down-bed into theup-bed air-stream, according to the invention;

[0046]FIG. 5 is an enlarged sectional view of a portion of the improvedWurster-type coating machine of FIG. 3, showing details of a dead-zone(shown in dashed line), the peripheral air-injection system, and flowarrows, according to the present invention;

[0047]FIG. 6 is an enlarged section view of a portion of the improvedWurster-type coating machine of FIG. 3 showing details of thenozzle-ramp and nozzle sleeve (showing a conventional straight-edgedpartition), and including flow arrows, according to the presentinvention;

[0048]FIG. 7 is a plan view of the peripheral air-injection systemshowing an inlet conduit, a ring manifold assembly including a pluralityof inwardly-directed discharge outlets, according to the invention;

[0049]FIG. 8 is a sectional view of the peripheral air-injection systemof FIG. 7, taken along the lines 8-8 of FIG. 7, according to the presentinvention;

[0050]FIG. 9 is a side view of the nozzle sleeve, according to theinvention;

[0051]FIG. 10 is a cross-sectional side view of the nozzle sleeve ofFIG. 9, according to the invention;

[0052]FIG. 11 is a partial-sectional side view of the nozzle-ramp,according to the invention;

[0053]FIG. 12 is a bottom view of the nozzle-ramp showing details of thethrough passages, according to the invention;

[0054]FIG. 13 is a sectional assembly view of the partition skirtshowing upper and lower sections, taken along the lines 13-13 of FIG.14, according to the invention;

[0055]FIG. 14 is a bottom view of the partition skirt, according to theinvention;

[0056]FIG. 15 is a partial-sectional side view of the partition skirt,according to the invention;

[0057]FIG. 16 is a plan view of a conventional orifice plate;

[0058]FIG. 17 is a sectional side view of the conventional orifice plateof FIG. 16, taken along the line 17-17 of FIG. 16;

[0059]FIG. 18 is a sectional view of a multi-partition coating machinehaving a central discharge assembly, according to another feature of thepresent invention, taken along the line 18-18 of FIG. 19;

[0060]FIG. 19 is a bottom view of the multi-partition coating machine ofFIG. 18, according to the invention, showing details of the nozzleassemblies and the central discharge;

[0061]FIG. 20 is an enlarged sectional side view of the centraldischarge assembly of FIG. 18, according to the invention, shown in aclosed position;

[0062]FIG. 21 is the central discharge assembly of FIG. 20, according tothe invention, shown in an open position; and

[0063]FIG. 22 is a sectional side view schematic of the improvedWurster-type bottom spray particle-coating machine similar to the oneshown in FIG. 3, and further including a split lower plenum whichdivides the air or gas stream and provides independent control of theflow of air or gas in the up and down bed regions of the Wurstermachine, according to the present invention.

DETAILED DESCRIPTION OF THE FEATURES OF THE INVENTION

[0064] By way of background and introduction, the present inventionprovides improvements relating to tablet flow and handling during thecoating process of an otherwise conventional Wurster-type bottom-sprayparticle coating machine (hereinafter referred to as “Wurster machine”).To better understand and appreciate the improvements of the presentinvention, a detailed description of the structure and operation of aconventional Wurster machine is first provided.

[0065] A. Description of a Conventional Wurster Machine

[0066] Referring to FIGS. 1 and 2 (labeled PRIOR ART), a conventionalWurster machine is shown, including a generally conical productcontainer 12, an orifice plate 14, a lower plenum 16, a central nozzle18 projecting upwardly through a central opening 20 located withinorifice plate 14, a nozzle sleeve 22, and a cylindrical partition 24.The product container 12 may be cylindrical or conical in shape and ismounted to an upper surface 26 of orifice plate 14, while the lowerplenum 16 connects with a lower surface 28 of orifice plate 14. Theupper end of the product container 12 is connected to an expansionchamber (not shown).

[0067] As shown in FIGS. 16 and 17, orifice plate 14 includes aplurality of orifices 30 which are arranged in such a manner as to allowair to flow (represented in FIGS. 1 and 2 by arrows 15) from the lowerplenum 16 through the orifices 30 and into the product container 12, tocreate a fluidized bed, as is described in greater detail below. Orificeplate 14 typically includes two sections of relative porosityconcentrically positioned around the central opening 20; an up bedsection 14A (also referred to as an “up bed plate”), and a down bedsection 14B (also referred to as a “down bed plate”). Air flow from thelower plenum 16 will behave differently in the up bed plate and the downbed plate. The air flow generated by a remote blower unit (not shown) isrestricted in a controlled and predictable manner so that a desiredfluidity of the solid particles located within the product container ismaintained. The up bed section 14A of the orifice plate (located underthe partition 24 and adjacent to the nozzle 18) includes a large numberof orifices 30 which allow a high volume of air from the lower plenum 16to pass through the orifice plate 14 and up through the partition 24 ata relatively high velocity. This upward flow of high velocity air iscalled the up bed (represented by arrows 33 in FIGS. 1 and 2) and isused to pneumatically transport the substrate vertically past the spraynozzle during each cycle.

[0068] The ring-shaped region outside of the partition within theproduct container 12 is referred to as the down bed (represented byarrows 37 in FIGS. 1 and 2). The down bed section 14B of the orificeplate 14 has a porosity (number, diameter and distribution of orifices30) which allows sufficient air flow to penetrate the substrate from thelower plenum 16 and to maintain the substrate located in the down bed innear-weightless suspension. This influx of air causes the particles 27to behave somewhat like a fluid and travel rapidly and freely downwardin the down bed 37 and subsequently be drawn horizontally through thetransition gap and redirected upwardly into the up bed. The amount ofair flow required to produce the near-weightless suspension of thesubstrate depends on the size and shape of the particular substrate. Ingeneral, tablets require significantly more air to produce thiscondition than pellets or fine particles and the orifice plate 14 mustbe designed accordingly.

[0069] The central nozzle 18 of the prior art Wurster machine 10 in FIG.1 is mounted through the central opening 20 of the orifice plate 14 sothat its discharge outlet is directed upwardly into the partition 24 andproduct container 12. The nozzle 18 ejects a highly atomized spray ofcoating liquid into the partition, creating a “coating zone” 32 which isusually shaped as a narrow volumetric ellipse. The exact shape of thecoating zone may be controlled according to the size of the substratebeing coated and the pattern density in the partition. The atomizedspray is discharged from the nozzle at about 300 meters per second.

[0070] Centrally and vertically mounted within the product container isthe cylindrical partition 24 whose diameter is about half the diameterof the product container 12 (as measured at the base of the productcontainer). The cylindrical partition 24 is positioned within theproduct container 12 adjacent to and above the central nozzle 18 so thatthe spray pattern discharged from the nozzle 18 extends into thepartition 24. As is understood by those skilled in the art, thepartition 24 is used to help direct particles 27 located within theproduct container 12 as they circulate throughout the Wurster machine,and further to protect the integrity of the spray pattern 32. Thepartition 24 also helps separate particles 27 moving upwardly in acentrally located up bed 33 through the coating zone 32 from particles27 that are falling back towards the orifice plate 14 in a peripherallyoriented down bed 37 with respect to the product container 12.

[0071] The partition 24 includes a lower edge or rim 25 and ispositioned within the product container 12 so that the lower rim 25 islocated a predetermined distance from the upper surface 26 of theorifice plate 14. The space (or “'transition gap”) that is createdbetween the lower rim 25 of the partition 24 and the upper surface 26 ofthe orifice plate 14 forms a “transition zone” where particles 27located in the peripheral down bed 37 are radially-inwardly drawnthrough a differential in pressure back into the central up-bed 33.

[0072] Conventional Wurster machines do not shield the relatively hardnozzle assembly 18 and will typically result in particle attrition andbreakage as the particles impact the nozzle assembly 18 as theyaccelerate from the down bed 37 to the up bed 33.

[0073] One improvement over the conventional Wurster machine regardingthe problem of particle-attrition and breakage due to an exposed nozzleassembly 18 is disclosed in commonly owned U.S. Pat. No. 5,236,503. Thispatent discloses a protective nozzle sleeve 22 which is a tubepositioned around the nozzle assembly 18. The nozzle sleeve 22 disclosedin U.S. Pat. No. 5,236,503 (and shown in FIG. 1 of the present patentapplication) prevents particles 27 from entering the spray pattern untilit is more fully developed, thereby increasing productivity.Furthermore, it keeps the substrate from encountering the extremely highcompressed air velocity at the tip of the spray nozzle thereby reducingparticle attrition and breakage. Unfortunately, some of the particlesaccelerating into the up bed 33 in the prior art Wurster machine (withthe improved protective nozzle sleeve 22) still impact the nozzle sleeve22, resulting in a measurable attrition rate. Such impact damage to theparticles 27 will only increase as the mass of the particles increases,such as when relatively heavy tablets are cycled through the coatingmachine.

[0074] Another problem with the prior art nozzle sleeve 22, as shown inFIG. 1 of the present patent application and also disclosed in U.S. Pat.No. 5,236,503 is that the sleeve itself is prone to trapping particles27 at the completion of the process, when fluidization air flow isstopped. Once trapped within the hollow nozzle sleeve 22, the particles27 will no longer circulate within the machine and may further interferewith the air flow around the nozzle, causing its spray pattern to bealtered and resulting in undesirable agglomeration of the particles.These trapped particles must be manually removed prior to loading asubsequent batch.

[0075] B. Convention Wurster Operation

[0076] In operation of the prior art Wurster machine shown in FIGS. 1and 2, a supply of particles 27 that are to be coated are placed withinthe product container 12. The size of the particles 27 are typicallybetween 100 and 2000 microns in diameter. The particular operatingparameters (such as the specific size, permeability, and arrangement ofthe orifices 30 in orifice plate 14, and the height of the transitiongap) of the Wurster machine will vary depending on the size and type ofparticles 27 being coated.

[0077] When the particles 27 are positioned within the product container12, a flow of filtered air is drawn from an air handling unit (notshown), the product container itself, and an expansion chamber (notshown) using an appropriate blower fan (also not shown) creating anegative pressure within the expansion chamber and the product container12. This negative pressure causes air (arrows 15) to be drawn upwardlythrough orifices 30 of orifice plate 14 from lower plenum 16. As the airis drawn into the product container 12 through orifices 30, it passesthrough both the up bed 33 and the down bed 37 (moving upwardly)penetrating and influencing the particles 27 located in the down bed 37,as shown in FIG. 1. The upward flow of air causes each particle 27 ofthe down bed 37 to effectively float or become suspended on a cushion ofair as the air flow finds its way upward into the product container 12to try to equilibrate the negative pressure in the expansion chamber.The “floating” particles 27 become “fluidized”, behaving more like afluid than a mass of solid particles. Air (arrows 15) from plenum 16 isalso drawn into the product container 12 through opening 20 and intonozzle sleeve 22 and further up into partition 24 adding to the up bedair stream 33.

[0078] After the flow of air from the lower plenum 16 fluidizes theparticles 27 within the product container 12, the central nozzle 18 isactivated to discharge a controlled spray pattern 32 of coating liquidupwardly into partition 24, as shown in FIGS. 1 and 2. The spray liquidgenerally is comprised of a solution, suspension or dispersion in wateror organic solvent (in some cases a molten liquid may be used) and isejected upwardly from the nozzle 18 at a high velocity around 300meters/second, depending on the particular type and size of particlebeing coated, the type of coating material used, and the desired coatingcharacteristics.

[0079] As the particles 27 rise rapidly upward in the high-velocity upbed air stream 33 created by the nozzle 18, they contact micro-atomizeddroplets of the coating liquid and become coated before slowing downwithin chamber (not shown). As the particles 27 continue to rise in thepartition 24 and into the expansion chamber, excess moisture from theapplied coating liquid evaporates.

[0080] The high-velocity air stream spouting from the upper end of thepartition 24 forces the particles 27 radially outwardly in the expansionchamber (as represented in FIG. 1 by arrows 35). Once away from theupstream lilt provided by the up bed 33, the particles 27 are influencedby gravity and fall within the product container 12 in the down bed 37,eventually reaching the orifice plate 14.

[0081] The high volume and velocity of the airflow into and through thepartition 24, combined with the high velocity of the air from the nozzle18, generates a very strong negative pressure in the transition zonelying adjacent to the nozzle 18 and the orifice plate 14 relative to themeasured pressure within down bed 37. This creates a pressuredifferential. The pressure differential draws the particles 27 that arelocated in the peripheral down bed 37 radially inwardly through thetransition zone and into the up bed 33. The up bed again accelerates theparticles 27 up into the partition 24 and through the coating zone 32.

[0082] The cycle is repeated for all particles 27 located within theproduct container 12, until a desired coating thickness is formed on allparticles of the batch.

[0083] C. Problems with the Conventional Wurster

[0084] As discussed in the Background section of this specification, theabove-described Wurster machine is generally effective at coating fineparticles within the product container. The longer the particles arekept circulating through the spray of the coating liquid, the greaterthe thickness of the coating on each particle, and the greater theconsistency between coated particles of the same batch. The Wurstermachine, however, fails to provide an accurate and predictabledistribution of coating on the particles as the particles increase inmass and size (such as when tablets or pills are cycled through themachine). The conventional Wurster machine includes three mainparticle-flow-related problems which are inherent in its design and areillustrated in FIG. 2 and described below.

[0085] A first flow-related problem of the Wurster machine relates to anuneven circulation flow of particles 27 located in the down bed 37resulting in the creation of a peripheral “dead zone” 40 (shown indashed lines in FIG. 2). During the coating process, the heavy tablets(or particles 27) within the down bed 37 exert pressure on the orificeplate 14, particularly along the peripheral wall of the productcontainer 12. Owing in part to this “loading”, a region of low-flow (andin some cases, no flow) is created along the outer perimeter of theproduct container 12 within an outer and lower section of the down bed37. The dead zone 40 generally extends approximately 50 mm above theorifice plate 14 and about 50 mm out into the product container. Thetablets (or particles 27) located within this peripheral dead zone 40 ofthe down bed 37 tend to slow down and even stop relative to tablets (orparticles 27) located radially inwardly within the down bed 37. Theseslower moving tablets (or particles 27) in the dead zone 40 fail tocirculate as often as the other tablets and will therefore have anadverse effect on the consistency of coating distribution betweentablets within the same batch. During the coating process, the surfaceproperties of the tablets change, and flow behavior in most instancesworsens (the flow of tablets slows down). This decrease in tablet flowtends to increase the probability that a slow or dead zone will formalong the perimeter of the base of the product container.

[0086] Again, referring to FIG. 2, a second flow-related problem of theprior art Wurster machine 10 relates to a misdirected flow of particles27. As a pressure differential is created in the transition zone betweenthe down bed 37 and the up bed 33 of the Wurster machine, some of theheavier particles 27 (e.g., tablets) lying close to the orifice plate 14fail to divert upwardly into the airstream of the up bed 33 and actuallyimpact against either nozzle 18, or nozzle sleeve 22 (if one is used).This impact path is represented by an arrow 42 in FIG. 2 and willinvariably increase attrition and breakage of the tablets.

[0087] A third flow-related problem of the prior art Wurster machine 10is that a percentage of tablets 27 located within the down bed andadjacent to the partition 24 are violently and traumatically pulled intothe up-bed 33 by a pressure differential, resulting in tablet attritionand breakage. This traumatic flow path is represented by an arrow 44 inFIG. 2.

[0088] Another problem associated with the prior art Wurster machinebecomes apparent after the coating process is complete and particles 27must be removed from the product container 12. The conventional processincludes hinging open the lower plenum 16 (and the orifice plate 14)from the lower portion of the product container 12 and literally dumpingthe coated particles 27 from the product container 12 into an awaitingcontainer. Not only does this crude emptying procedure expose theimmediate environment (including workers) to potenially hazardousmaterials (such as drug residue), it also esposes the freshly coatedparticles and the interior surfaces of the product container and orificeplate to possible contamination.

[0089] Also, the conventional Wurster machine is difficult to clean,usually requiring hinging open the lower plenum 16 (as described above)and spraying a cleaning solution throughout the product container andexpansion chamber, allowing the waste cleaning fluid (which iscontaminated and potentially hazardous) to pour from the machine throughthe open lower plenum 16. As during the above-described particleemptying procedure, this prior art cleaning process introducespotentially hazardous materials to the immediate environment which aredifficult to handle and contain.

[0090] Finally, the prior art nozzle sleeve 22, described in commonlyowned U.S. Pat. No. 5,236,503 fails to adequately prevent particlesimpacting its surface, and further is prone to accumulating and trappingparticles at the completion of the coating process. U.S. Pat. No.5,236,503 is hereby incorporated by reference into this specification.

[0091] D. Description of the Present Invention

[0092] Referring now to FIG. 3, a Wurster machine is disclosed includingimproved features according to the present invention. The improvedfeatures solve the above-discussed problems of the conventional Wurstermachine so that the improved Wurster machine may be used to accuratelyand efficiently coat heavier particles 27, such as tablets and pills(hereinafter referred to as “tablets”).

[0093] 1. Air-Injection Manifold

[0094] As discussed above in the background section of thespecification, one problem inherent in the design of the conventionalWurster machine is the existence of dead zones, wherein tablets 27become stagnant and cycle fewer times than other tablets in the samebatch.

[0095] According to a first feature of the invention, as shown in FIGS.3, 4, 5, 7 and 8, an air-injection manifold 50 is provided around thelower end of the product container 12 adjacent to the orifice plate 14and which overcomes the problems associated with the creation of deadzones in prior art coating machines.

[0096] As shown in FIGS. 5,7 and 8, ring manifold 50 is circular andincludes an inner surface 52 having a lower edge 54, an upper surface 56having a circumferential channel 58 located immediately adjacent to theinner surface 52, a bottom surface 59, and a plurality of openings 60evenly spaced along the inner surface 52 immediately adjacent to thelower edge 54. Openings 60 are directed radially inwardly towards thecenter of the circular manifold 50. Each of the openings is in fluidcommunication with an internal circumferential conduit 62 which is shownin section in FIGS. 3, 4, 5, and 8. An inlet conduit 64 (FIG. 7)connects with the internal conduit 62 so that air supplied underpressure to inlet conduit 64 flows within conduit 62 and dischargesevenly throughout the plurality of openings 60 around the inner surface52. This discharge of air flow from openings 60 results in a radiallydirected flow of air (airflow from selected openings 60 is representedby arrows 65 in FIG. 7).

[0097] Since manifold 50 is intended to be used in a clean environment,it is preferably made as an assembly of parts which may be selectivelydisassembled to access and clean all surfaces. To this end, internalconduit 62 is preferably formed by fitting a conduit ring 66 and abottom sealing ring 68 with an outer main ring 70, as shown in FIGS. 3,4, and 8.

[0098] According to the invention, manifold 50 is positioned between aside wall 72 of product container 12 and orifice plate 14, as shown inFIGS. 3, 4, and 5, and is sized and shaped so that a lower end 74 ofside wall 72 snugly fits within the circumferential channel 58 and formsa smooth transition between an inner surface 75 of side wall 72 andinner surface 52 of manifold 50. Inner surface 52 of ring manifold 50preferably angularly aligns with the conical product container 12.Bottom surface 59 of manifold 50 is mounted to the upper surface 26 oforifice plate 14 so that the openings 60 of manifold 50 lie immediatelyadjacent upper surface 26 of orifice plate 14. With this arrangement,according to the invention, air (or any fluid, including cleaningliquids or rinse water) that is introduced under pressure into inletconduit 64 will discharge through openings 60 in a radially inwarddirection across the upper surface 26 of orifice plate 14. Asillustrated in FIG. 5, this inwardly directed blast of air 65 frommanifold 50 effectively prevents the dead zone 40 from forming byforcing all tablets 27 located in this region to move horizontallytowards the transition zone and nozzle 18 (as represented by arrows 61in FIG. 5). Tablets 27 are not shown in dead zone 40 of FIG. 5 forclarity so that air flow arrow 65 can be seen and understood. Accordingto the invention, manifold 50 keeps all of the heavy tablets movingevenly from down bed 37 to up bed 33 so that an otherwise conventionalWurster machine may he used to coat heavy tablets 27 without creating aperipheral dead zone 40 within the product container 12. Additionally,as discussed in greater detail below, the volume and velocity of the airfrom openings 60 may be adjusted to compensate for changing tabletsurface flow properties during the application of coating material tothe tablets. This manifold air adjustability is independent of theprocess air flow through the orifice plate in the up bed 33 and down bed37 regions.

[0099] The effectiveness of introducing a radially directed blast of airfrom the periphery of the product container 12 along the upper surface26 of the orifice plate 14 can be appreciated through the illustrationsof FIGS. 4 and 5. Arrow 65 of FIG. 5 represents the force of theradially inwardly directed blast of air while arrow 78 represents the“loading” or force exerted by the tablets 27 located in the down bed 37on orifice plate 14. The horizontal force 65 generated by the dischargedair from openings 60 of manifold 50 move the lower tablets 27 of thedown bed 37 radially inwardly as shown by arrow 61, to effectively makeroom for other tablets 27 of down bed 37 and to keep all of the tablets27 moving in a smooth and consistent flow from down bed 37 to up bed 33.

[0100] Referring to FIG. 4, a representative tablet 27 moves on aradially inward path (arrow 65) in response to the radially inwardlydirected flow of air discharged by openings 60 of manifold 50. Thehorizontal air injection flow created by manifold 50 cooperates with theconventional flow of air passing through openings 30 of orifice plate 14to maintain fluidization of tablets 27.

[0101] The air pressure used to feed manifold 50 will vary depending onthe size and shape of the tablets 27 being coated, their changing flowproperties during a coating process, the size and shape of theparticular product container 12, the particular configuration of theorifice plate 14, and other operational and structural parameters of themachine. The air pressure measured in one operational example was about20 pounds per square inch (p.s.i.).

[0102] According to another aspect of the invention, the supplied airpressure may be controlled so that the velocity of the air dischargedfrom openings 60 will vary at predetermined time periods during acoating procedure. Tablets 27 are typically provided with a lubricant ontheir surface which allows them to flow easily throughout the coatingmachine during the first few minutes of the coating process. As thecoating is applied, however, the surface of each tablet 27 tends tobecome a bit tacky, resulting in a slower descent rate in the down bed37. By controlling the radially directed air injection (independent ofthe flow of air through the orifice plate) over time, the flowresistance caused by the “tackiness” of tablets 27 in the down bed 37can be accounted for and minimized, resulting in a consistent down bed(and up bed) behavior. For most instances, during the coating process,the air flow through outlets 60 of manifold 50 may be controlled togradually increase in velocity and volume.

[0103] 2. Nozzle Sleeve

[0104] Referring to FIGS. 3, 4, 9, and 10, a nozzle sleeve 90 is shownaccording to a second feature of the present invention. Nozzle sleeve 90is hollow and includes a generally cylindrical base portion 92, having acircular bottom edge 94, a truncated conical upper portion 96 having acircular upper edge 98 (defining an upper opening 99), and a centralhollow passage 100. Nozzle sleeve 90 is sized and shaped to fit aroundnozzle 18 (see FIGS. 3 and 6). Passage 100 and the diameter of upperopening 99 may be sized with respect to the diameter of nozzle 18 sothat air may flow freely up through passage 100 and through upperopening 99, adjacent to nozzle 18 during the operation of the machine,as described below. By directing air through the passage 100 of nozzlesleeve 90 in this manner, the discharge of air through upper opening 99adjacent to nozzle 18 may assist in shaping and controlling the shapeand characteristics of the spray pattern generated by the nozzle.Alternatively, the upper opening 99 may be sized to tightly receivenozzle 18 so that no air (or minimal air) will pass through nozzlesleeve 90 during the operation of the machine. Nozzle sleeve 90 issecured in position with bottom edge 94 abutting against orifice plate14.

[0105] Once in position around nozzle 18 within product container 12,and during the coating operation, nozzle sleeve 90 serves threefunctions. First, nozzle sleeve 90 protects tablets 27 from directlyimpacting the harder surfaces of nozzle 18 during operation, asdescribed below. Second, conical upper portion 96 is shaped toaccommodate the natural flow of tablets 27 as they are drawn into the upbed 33 from the down bed 37, as shown in FIG. 3. Third, the hollowpassage 100 and the conical upper portion 96 direct air from the lowerplenum 16 to assist in shaping the up bed 33 and the coating zone 32.

[0106] Nozzle sleeve 90 is preferably made from a strong, somewhatresilient plastic, such as a PTFE or Delrin, or an appropriate rubber,such as silicone, and is preferably adapted to be easily installedwithin a coating machine and quickly and easily replaced to minimizesetup time. The particular dimensions and shape of nozzle sleeve 90 mayvary according to particular parameters of the coating machine, as isunderstood by those skilled in the art. The nozzle sleeve may beseparate or integrated into the nozzle ramp, and may be solid orperforated to permit air flow in proximity to the spray nozzle.

[0107] 3. Nozzle Ramp

[0108] As mentioned above, a problem with the conventional Wurstercoating machine is that heavier tablets 27 are traumatized during theirtransition from the down-bed 37 of the product container 12 to thecentral up-bed 33 through the partition 24. The heavier tablets used ina conventional Wurster machine may also be damaged by impacting thenozzle assembly during bed transition. The up bed 33 moves much fasterthan the peripheral down bed 37 within the product container and astrong negative pressure is developed around the center of the orificeplate and within part of the partition. As discussed above, thisnegative pressure rapidly draws tablets 27 from the peripheral down bed37 radially inwardly along a horizontal path into the airstream of theup bed 33. Owing to the mass of the tablets 27, the horizontal componentof the inertia imparted to the tablets 27 by the negative pressure isoften too great for the upwardly moving airstream of the up bed 33 tocompletely vertically redirect the horizontally moving tablets 27 beforesome of the tablets 27 impact the centrally located nozzle assembly 18,nozzle sleeve 90 (if one is used) and/or other tablets 27 entering fromopposing directions along the orifice plate 14.

[0109] Referring to FIGS. 4, 6, 11, and 12, a nozzle ramp 102 is shown,according to a second feature of the present invention, which overcomesthe above-mentioned problem of tablets 27 impacting nozzle 18 as theyenter into the up bed 33. Nozzle ramp 102 includes a circular base 104having a perimeter 106, a hollow cylindrical center 108 having a sidewall 110 and a top edge 112, and an arcuate ramp surface 114 (having ashape that is similar to a cusp) positioned between perimeter 106 ofbase 104 and top edge 112.

[0110] As shown in FIG. 3, nozzle ramp 102 is centrally positionedwithin product container 12 with its base 104 mounted flush againstupper surface 26 of orifice plate 14 (using bolts, for example). Nozzleramp 102 may be use within the product container with or without anozzle sleeve. Should a nozzle sleeve be used, hollow center 108 ispreferably sized and shaped to accommodate both nozzle 18 and nozzlesleeve 90, described above, or alternatively, a conventionally shapednozzle sleeve 22, such as the one shown and described in U.S. Pat. No.5,236,503. Further, nozzle ramp 102 may be formed with an integralnozzle sleeve (not shown), however, it is preferred that nozzle sleeveremain as a separate and attachable part so that tablet and air flowcharacteristics can be better controlled.

[0111] Nozzle ramp 102 further includes a plurality of verticallydisposed passages 116 which are preferably arranged in concentric ringspassing between the arcuate ramp surface 114 and circular base 104, asshown in FIG. 11 (in section). These vertical passages 116 are sized andshaped to exactly align with corresponding openings 30 of a conventionalorifice plate 14, which is shown in FIGS. 18 and 19, so that air passingthrough openings 30 from lower plenum 16 freely passes through thealigned vertical passages 116 of nozzle ramp 102 and becomes dischargedat an upper end of each respective passage 116 along arcuate rampsurface 114. Passages 116 are preferably either equal to or larger thanthe corresponding openings 30 of orifice plate 14. Orifice openings 30are sized to control the flow of air within passages 116.

[0112] The purpose of the nozzle ramp 102 is to direct air from thelower plenum 16 along a curved circular ramp surface so that the fastmoving, horizontally driven tablets 27 can be atraumatically coerced tofollow a vertical trajectory using a cushion of air. As discussed below,the use of nozzle ramp 102 will minimize undesirable impacting oftablets 27 against nozzle 18 or nozzle sleeve 90. The conical upperportion 96 of nozzle sleeve 90, according to the above described featureof the present invention, preferably generally aligns with arcuate rampsurface 114 of nozzle ramp 102, as shown in FIGS. 3 and 4 so thattablets 27 being diverted to the up bed by the nozzle ramp 102 mayfollow a less severe arcing path (as shown as arrow 118 in FIG. 4) andstill avoid impacting any portion of nozzle sleeve 90.

[0113] 4. Partition Skirt

[0114] The tablets 27 located in the down bed 37 (FIG. 1) of a Wurstermachine generally move downwardly at a rate of one meter inapproximately 10 to 30 seconds. When these tablets 27 are drawn into theup bed 33, where they accelerate to approximately 5 to 10 meters persecond, they encounter an atomizing air velocity of about 300 meters persecond. This violent change in velocity causes great transitional traumaand high shear to the relatively fragile tablets 27 and will likelyincrease the attrition rate of the tablets of the batch early in thecoating process. The relatively sharp lower edge 25 of the conventionalpartition 24 which separates the down bed 37 and the up bed 33 onlyexacerbates the transitional trauma to the tablets 27 as they are drawninto the fast moving upward current of the up bed.

[0115] Referring to FIGS. 3, 4, 13, 14, and 15, a partition skirt 120according to another feature of the present invention is provided at thelower edge 25 of partition 24 which overcomes the problems relating totransitional-trauma of the tablets 27 entering the high velocity up bed33. Skirt 120 is a ring-shaped cone having a angled outer surface 122 atangle A (FIG. 13), a cylindrical inner bore 124, and a lower surface126. Bore 124 is sized and shaped to snugly receive partition 24 so thatskirt 120 may be secured to the lower end of partition 24 (preferably ina manner that allows skirt 120 to be quickly and easily removed frompartition 24 if necessary). The diameter of bore 124 is preferably equalto or slightly larger than the outside diameter of partition 24.

[0116] Lower surface 126 of skirt 120 is preferably beveled at aprescribed angle B, as shown in FIG. 13, forming an inwardly directedfunnel shape which extends between angled surface 122 and bore 124.Lower surface 126 functions as an “on-ramp” allowing tablets 27 adjacentto skirt 120 to “get up to speed” before entering the high velocity upbed 33. The exact degree of angle B of the lower surface 126 will varydepending on the size, shape and weight of the tablets 27, thedimensions of the product container 12, coating characteristics, as wellas other structural and operational factors and parameters, but angle Bwill generally be in the range of 15 and 30 degrees. Lower surface 126preferably includes a rounded outer edge 129, as shown in FIG. 15.

[0117] Tablets 27 located in down bed 37, in particular adjacent to thepartition 24, will be directed away from partition 24 as they descenddown bed 37 by angled surface 122 until they reach lower surface 126 atwhich point tablets 27 will gradually pick up speed due to the pressuredifferential created by the up bed 33 and move inwardly along lowersurface 126. As tablets 27 move inwardly along lower surface 126, theywill gradually accelerate before “falling upward” into the high velocityup bed 33. Lower surface 126 allows tablets 27 to gain speed and therebyreduces the effects of shear and other mechanical stresses created bythe gradient between down bed 37 and up bed 33. This less traumaticintroductory path into up bed 33 is represented by the arrow 127 in FIG.4.

[0118] Skirt 120 is preferably made from a strong resilient plastic,such as PTFE or Delrin, but may be made from any appropriate materialincluding other plastics, rubber, and metal, such as stainless steel. Tosimplify the manufacturing of skirt 120 and to introduce versatility,skirt 120 may be made from two pieces, as shown in FIG. 13 including alower ramp ring 128 and an upper conical sleeve 130. Ramp ring 128 andconical sleeve 130 may be formed separately (milled or molded) andthereafter secured along a mating surface 132 to each other using anyappropriate bonding, fastening or welding technique, as understood bythose skilled in the art. In one embodiment, the two pieces making upskirt 120 are attached in an easily removable manner so that one of manyconical sleeves 130 having a particular angle A may be fitted with oneof many ramp rings 128 having a particular angle B. Although it ispreferred that skirt 120 be provided as a part to be attached to thelower portion of partition 24, it is also contemplated that skirt 120and partition 24 be made integrally as a single piece.

[0119] The purpose of skirt 120 is to provide a somewhat horizontalsurface (lower surface 126) on which particles 27 may gradually andatraumatically accelerate in the up bed 33. The lower surface 126 isformed between the wall of partition 24 and the angled surface 122, asshown in FIG. 4. Some prior art Wurster machines use a partition thatincludes an outwardly flared lower end. This flared lower end does notdefine or otherwise establish an angled surface 122 (or any horizontalsurface between the down bed and the up bed). The purpose of the priorart flared lower end is to allow particles to be drawn into the up bedwithout “crowding” the nozzle and disrupting the spray pattern. Althoughthe flared lower end of the prior art partition forces particles locatedin the down bed outwardly towards the peripheral wall of the productcontainer, the particles are still traumatically drawn into the up bedbecause there is no angled (or generally horizontal) surface 122, as inthe present invention.

[0120] As described above and according to the invention, skirt 120diverts tablets 27 located in the down bed 37 away from lower edge 25 ofpartition 24 and provides an inclined ramp (angled lower surface 126) sothe adjacent tablets 27 are not harshly and traumatically drawn into theup bed 33. Skirt 120 also helps channel the “loading” of down bed onorifice plate 14 outwardly near the periphery of the product container12. By doing this, a larger transition zone is created. Tablets 27located under skirt 120 are more easily suspended by the air flowingthrough orifice plate 14 from lower plenum 16 because there is less orno downward force exerted on them by tablets 27 located higher in thedown bed 37. The result is that tablets 27 move more easily and lesstraumatically from the down bed 37, through the transition zone, andinto the up bed. To maximize this effect, it is preferred that thedistance between the outermost point of skirt 120 and the wall of theproduct container 12 (represented by arrow C in FIG. 4) be approximatelyequal to the distance between the lowermost point of skirt 120 and theupper surface 26 of orifice plate 14 (represented by arrow D in FIG. 4).

[0121] The particular dimensions and shapes of all the above-describedcomponents of the improved machine have a mathematical relationshipwherein the particular parameters of one component are related anddetermined by the particular parameters of another component, and thecharacteristics of the particles being coated, and the desired coatingresults. Finite analysis techniques may be used to establish therelationship between the components.

[0122] 5. Central Discharge

[0123] A rate limiting factor in the productivity of the Wurster processis the relatively narrow coating zone which, in turn, restricts thediameter of the partition 24 to about nine inches. To increase theefficiency and the productivity of a Wurster machine, the size of theproduct container may be increased if multiple partitions and nozzlesare used. For instance, to operate efficiently, a Wurster machine havingan eighteen inch diameter product container uses a nine inch diameterpartition. However, a thirty two inch Wurster may require three, nineinch diameter partitions spaced evenly within the product container, anda forty six inch Wurster may include six or seven, nine inch diameterpartitions.

[0124] Referring to FIGS. 18, 19,20 and 21, an improved productdischarge assembly 140 of the present invention is shown, suitable forefficiently removing tablets 27 (or particles, powders, granules,pellets, or grains) in a sealed and controlled manner from a productcontainer 12 of the type having multiple partitions 24 and nozzles 18.An exemplary coating machine 142 having three nozzles 18 and threepartitions 24 is shown and described herein to explain the structure andoperation of discharge assembly 140, according to the invention. Thedischarge assembly 140, according to the invention, may be used with anymulti-partition/nozzle Wurster machine or with product containers inconventional fluidized bed drying or spray granulating equipment. Sheabove-described features of the invention including the nozzle ramp, thelower skirt assembly, and the air-discharge manifold are not shown inFIGS. 18 and 19 for clarity. Any and all of the features described inthis specification may be used in any combination in a coating or dryingmachine.

[0125] Referring to FIG. 18, partition 24 may include an outwardlyflared upper end 145, as shown, having a shape which allows particles 27to quickly exit up bed 33 (of partition 24) and enter down bed 37without substantially impacting the side wall of partition 24. Thespecific shape of the flare is preferably cusp shaped, but flared upperend 145 may alternatively be conical in shape. Flared upper end 145allows particles 27 to disperse from partition 24 without particleattrition or breakage and encouraging a smooth transition of theparticles from the up bed 33 to the down bed 37. Flared upper end 145may be used in single partition machines, described above, ormulti-partition machines, (only one of the partitions 27 of FIGS. 18 and19 is shown with a flared upper end 145 to illustrate the flaredfeature). Also, the upper end of partition 27 may include a resilient orimpact absorbent material, such as a rubber or suitable plastic to helpminimize particle or tablet attrition. The absorbent material (not shownin the figures) may be in the form of a coated layer or an attachablesleeve or layer.

[0126] For these larger coating machines, once a coating process iscomplete for a particular batch of tablets 27, the tablets are typicallyremoved by opening a pivotal lower end of the machine and literallydumping the contents of the product container 12 into an awaitingcontainer (not shown). As described above, this prior art process forremoving coated tablets 27 may easily introduce contamination to boththe tablets and the interior portions of the machine 10, as well asexpose workers to potentially hazardous materials.

[0127] The improved machine 142, shown in FIGS. 18 and 19 includes adischarge conduit 144 which extends from an orifice plate 146 to anaccessible location remote from the machine 142. Orifice plate 146 issimilar to the above-described orifice plate 14, except that it isdesigned for three nozzles 18 positioned 120° apart from each other andtherefore includes three large openings (not shown). According to thisfeature of the invention and referring to FIGS. 18-21, orifice plate 146further includes a central discharge opening 150 which is sized andshaped to accommodate discharge conduit 144, as shown in FIGS. 20 and21. A conical conduit cover 152 is movably fitted above orifice plate146 in alignment with central discharge opening 150. The conical cover152 is oriented with its apex directed upward and is movable between twopositions, a sealed position (shown in FIG. 20) and an open position(shown in FIG. 21). A linear actuator 154 is connected to conical cover152 by one or more armatures 155 (only one armature I55 is shown in thefigures) so that when activated, actuator 154 linearly displacesarmature 155 which, in turn, displaces conical cover 152 between thesealed position (FIG. 20) and the open position (FIG. 21). Actuator 154may be any appropriate type, such as an electromagnetic actuator (e.g.,a solenoid), a pneumatically driven cylinder, an hydraulically drivenram device, or a mechanically operated device such as a system of cablesand/or levers (not shown). The purpose of actuator 154 is to open orclose conduit cover 152, as desired, and as further discussed below.

[0128] When conduit cover 152 is closed, as shown in FIG. 20, tablets 27remain sealed within product container 12 and, if coating machine 142 isoperating, tablets 27 will circulate in a manner similar to the earlierdescribed single partition coating machine 10 (represented by arrows 153of FIG. 20), without being obstructed or otherwise affected by conduitcover 152. Conical cover 152 will function as a divider, evenlydirecting the tablets 27 of a central common down bed to each of thethree nozzles 18.

[0129] When the coating process is complete (or it is otherwise desiredto remove tablets 27 from product container 12), actuator 154 isactivated causing conduit cover 152 to be vertically displaced aboveorifice plate 146, as shown in FIG. 21, forming a gap 156 between theperimeter of discharge opening 150 and the perimeter of conduit cover152. The opening of conical cover 152 exposes an open end of dischargeconduit 144 which causes tablets 27 to be drawn into discharge conduit144, as represented by arrows 158 of FIG. 21, as they continue tocirculate within the coating machine 142 between the down bed and the upbed, as described above in connection with earlier features of theinvention. Tablets 27 are preferably drawn into discharge conduit 144using a pressure differential between the product container 12 and thedischarge conduit (i.e., by creating a vacuum within discharge conduit144). The tablets 27 drawn into discharge conduit 144 (represented byarrow 160 of FIG. 21) exit product container 12 and may be collected inan a waiting container (not shown), while remaining in a sealed andcontrollable environment.

[0130] The central discharge assembly described above and shown in FIGS.18-21, is intended to be used only for larger coating machines whichrequire three or more nozzles 18 and partitions 24. The centraldischarge assembly is preferably located in the center of the productcontainer 12, but could be located elsewhere along orifice plate 14. Thecentral discharge feature of the invention may be used alone or incombination with any of the other features of this invention and furtherwith any conventional coating machine, or other type oftablet-processing/handling machine including fluidized bed granulatingand/or drying machines. The central discharge assembly is preferablyused in combination with the above-described compressed air manifoldwhich would assist in forcing product located along the periphery of theproduct container 12 inwardly towards the central discharge assembly.This assistance to the tablets is particularly useful near the end ofthe discharge process when few tablets remain in the product containerand the fluidization air has been reduced to a minimum or stoppedcompletely. The central discharge system is not limited to use withtablets, but is equally effective with smaller substrates such aspellets, granules, crystals or powders.

[0131] 6. Cleaning

[0132] After a predetermined period of cycle time, the interior surfacesof the multi-partition coating machine 142 described above and shown inFIGS. 18-21 must be cleaned using a cleaning fluid. As described above,it is known to position spray nozzles within the expansion chamber andproduct container and apply a cleaning fluid along most of the interiorwall surfaces, rinsing drug residue (or other materials) and othercontaminants down towards the orifice plate 14. However, much of thedrug residue includes relatively large particles which are too large topass through the openings 30 of orifice plate 14 or through a finescreen (not shown), if one is used in combination with the orifice plate30. These large particles wash down the wall surfaces of the productcontainer and become deposited onto upper surface 26 of the orificeplate 14 or fine screen, typically in the dead zone 40 (see FIG. 2 PRIORART), while the waste cleaning fluid passes through the fine screen andthe lower plenum 16 as it drains.

[0133] In accordance with another feature of the present invention,referring to FIGS. 3, 7 and 18-21, during the cleaning process, cleaningfluid may be injected under high pressure through inlet conduit 64 andinner conduit 62 so that the fluid discharges through openings 60 andacross orifice plate 14 (or across a fine screen not shown, which may beplaced on top of orifice plate 14). The radially discharged cleaningfluid will force any of the large particles deposited on orifice plate14 to also move radially inwardly towards the central dischargeassembly. According to the invention, conical cover 152 is moved to itsopen position (as shown in FIG. 21) during the cleaning process so thatthe now exposed central discharge conduit 144 may serve as a centraldrain for any of the larger particles unable to pass through theopenings 30 of the orifice plate.

[0134] While the use of the compressed air manifold in combination withthe central discharge is described for use with a Wurster tablet coatingmachine, these components may also be installed and used in the samemanner for removing powders, granules, and/or coated particles fromconventional fluidized bed drying and/or spraying granulating equipment.Also, each of the above-described improvements of this invention may beused alone or in any combination with each other in any type ofvertical-spray fluidized bed granulating machines or drying granulatingmachines, if appropriate.

[0135] 7. Split Plenum Arrangement

[0136] According to another embodiment of the invention, referring toFIG. 22, a Wuster machine 10 is shown similar to the one shown in FIG. 3and described above, however the lower plenum 16 now includes aslit-plenum arrangement. The split plenum arrangement includes a centralconduit 170 and a peripheral conduit 172. The central conduit 170extends from the lower surface of orifice plate 14 and is sized andshaped to generally direct air or gas upwardly through nozzle sleeve 90,nozzle ramp 102 (through passages 116) to define the up-bed flow of airin the product container 12. The central conduit 170 is connected to asource of air flow (pressurized gas or appropriate fan) not shown, andfurther includes a metering system 174 for measuring the speed,pressure, volume, humidity, and/or temperature of the passing centralup-bed air flow. The metering system 174 is connected to the source ofair flow (not shown) so that an air flow having desired flowcharacteristics can be achieved and maintained using conventionalfeedback controlling subsystems, for example.

[0137] The peripheral conduit 172 is sized and shaped to supply air flowthrough the remaining exposed portion (everything around the centralconduit) of the orifice plate 14, thereby controlling the fluidized bedcharacteristics of the down-bed and the transition bed of the productcontainer. Similar to the central conduit 170, the peripheral conduit172 is connected to a dedicated source of air flow (not shown, andsimilarly includes a metering system 176 for measuring the speed,pressure, volume, humidity, and/or temperature of the passing peripheralair flow. The metering system 176 may be similar to the central air-flowmetering system 174 and may similarly be used as a feedback controllingsystem to maintain air-flow having desired preset characteristics.

[0138] By separating the flow through the lower plenum 16 into centraland peripheral regions, up-bed, down-bed, and transitional-bed flow andfluidization characteristics may be more accurately and independentlycontrolled.

What is claimed is:
 1. An improved Wurster-type fluidized bed apparatusfor applying a coating liquid onto the surface of particles, saidapparatus being of the type including a vertically disposed generallycylindrical product container having a peripheral wall, at least onecylindrical partition defining a centrally located up-bed region and aperipherally located down-bed region, said product container furtherincluding an upper end connected to an expansion chamber, a lower endincluding an orifice plate having a plurality of openings for passage offluidized air, a nozzle centrally located through said orifice plate andbeing adapted to generate a spray of said coating liquid upwardly intosaid up-bed, wherein said particles located within said productcontainer circulate upwardly through said partition and said spray ofcoating liquid, between said up-bed and said down-bed, said improvementcomprising: a plurality of inwardly directed discharge jets positionedalong the periphery of said cylindrical product container and generallyadjacent to said orifice plate; and a source of pressurized gasconnected to said plurality of discharge jets for selectively providinga radially inwardly directed flow of gas through said particles of saiddown-bed so that said particles located adjacent to said peripheral wallof said product container are displaced radially inwardly along saidorifice plate towards said nozzle and said up-bed.
 2. The improvedWurster-type fluidized bed apparatus, according to claim 1, furthercomprising a manifold positioned adjacent to said peripheral wall ofsaid product container, said manifold providing fluid communicationbetween said source of pressurized gas and at least two of saiddischarge jets.
 3. The improved Wurster-type fluidized bed apparatus,according to claim 1, wherein said discharge jets are evenly spacedabout the peripheral wall of said product container.
 4. The improvedWurster-type fluidized bed apparatus, according to claim 1, wherein saiddischarge jets comprise a single annular discharge slot positionedaround said peripheral wall of said product container and generallyadjacent and parallel to said orifice plate.
 5. The improvedWurster-type fluidized bed apparatus, according to claim 2, wherein saidmanifold includes a circular conduit ring which defines a circular gasconduit, and a circular bottom sealing ring, said discharge jets beingformed between said bottom sealing ring and said conduit ring, saiddischarge jets being formed in fluid communication with said gasconduit.
 6. The improved Wurster-type fluidized bed apparatus, accordingto claim 1, wherein said gas discharged through said discharge jets isair.
 7. The improved Wurster-type fluidized bed apparatus, according toclaim 1, wherein said gas discharged through said discharge jets is aninert gas.
 8. The improved Wurster-type fluidized bed apparatus,according to claim 7, wherein said inert gas discharged through saiddischarge jets is nitrogen.
 9. An improved Wurster-type fluidized bedapparatus for applying a coating liquid onto the surface of particles,said apparatus being of the type including a vertically disposedgenerally cylindrical product container having a peripheral wall, atleast one cylindrical partition having a lower rim and defining acentrally located up bed region and a peripherally located down bedregion, said product container further including an upper end connectedto an expansion chamber, a lower end including an orifice plate having aplurality of openings for passage of fluidized air, a nozzle centrallylocated through said orifice plate and being adapted to generate a sprayof said coating liquid upwardly into said up bed, wherein said particleslocated within said product container circulate upwardly through saidpartition and said spray of coating liquid, between said up bed and saiddown bed, said improvement comprising: a partition having an outer wallsurface and an outwardly directed flared lower end defining a lower rim,and a transition surface extending between said lower rim and saidup-bed, so that particles located within said down-bed adjacent to saidouter wall surface are urged outwardly away from said up-bed untilreaching said lower rim, at which point, said particles areatraumatically and gradually directed inwardly along said transitionsurface into said up-bed.
 10. The improved Wurster-type fluidized bedapparatus, according to claim 9, wherein said outward directed flaredlower end is conically shaped around said outer wall surface of saidpartition flared outwardly at an angle between 1° and 50° as measuredfrom the vertical outer wall surface.
 11. The improved Wurster-typefluidized bed apparatus, according to claim 9, wherein said transitionsurface is rounded, similar to a U-shape.
 12. The improved Wurster-typefluidized bed apparatus, according to claim 9, wherein said transitionsurface is an inwardly and upwardly directed ramp surface and includesan incline of between 105° and 120°, as measured from the vertical outerwall surface.
 13. The improved Wurster-type fluidized bed apparatus,according to claim 12, wherein said outwardly directed flared lower endramp surface is spaced from said peripheral wall of said productcontainer a first distance, and said ramp surface is positioned a seconddistance above said orifice plate, said first distance being generallyequal to said second distance.
 14. The improved Wurster-type fluidizedbed apparatus, according to claim 12, wherein said outwardly directedflared lower end ramp surface is spaced from said peripheral wall ofsaid product container a first distance, and said ramp surface ispositioned a second distance above said orifice plate, said firstdistance being greater than said second distance.
 15. The improvedWurster-type fluidized bed apparatus, according to claim 12, whereinsaid outwardly directed flared lower end ramp surface is spaced fromsaid peripheral wall of said product container a first distance, andsaid ramp surface is positioned a second distance above said orificeplate, said first distance being less than said second distance.
 16. Animproved Wurster-type fluidized bed apparatus for applying a coatingliquid onto the surface of particles, said apparatus being of the typeincluding a vertically disposed cylindrical product container having aperipheral wall, at least one cylindrical partition defining a centrallylocated up bed region and a peripherally located down bed region, andproduct container further including an upper end connected to anexpansion chamber, a lower end including an orifice plate having aplurality of openings for passage of fluidized air, a nozzle centrallylocated through said orifice plate and being adapted to generate a sprayof said coating liquid upwardly into said up bed, wherein said particleslocated within said product container circulate upwardly through saidpartition and said spray of coating liquid, between said up bed and saiddown bed, said improvement comprising: a generally cusp shaped rampsurface centrally positioned around said nozzle and directed upwardlytowards said partition, said cusp shaped ramp surface being shaped todirect particles moving generally horizontally across said orifice platefrom said down-bed upwardly into said partition and said up-bed.
 17. Theimproved Wurster type particle coating apparatus according to claim 16,further comprising a plurality of vertically directed air passages whichalign with said openings of said orifice plate so that a portion of saidfluidized flow of air passes through said air passages and creates acushion of air along said cusp shaped ramp surface, said cushion of airdirecting particles upwardly from said down bed and preventing impactwith said nozzle.
 18. An improved Wurster-type fluidized bed apparatusfor applying a coating liquid onto the surface of particles, saidapparatus being of the type including a vertically disposed cylindricalproduct container having a peripheral wall, at least one cylindricalpartition defining a centrally located up bed region and a peripherallylocated down bed region, said product container further including anupper end connected to an expansion chamber, a lower end including anorifice plate having a plurality of openings for passage of fluidizedair, a nozzle centrally located through said orifice plate and beingadapted to generate a spray of said coating liquid upwardly into said upbed, wherein said particles located within said product containercirculate upwardly through said partition and said spray of coatingliquid, between said up bed and said down bed, said improvementcomprising: a discharge opening located within said orifice plate, saiddischarge opening including a conduit positioned below said orificeplate, said discharge opening and said conduit being sized and shaped toselectively discharge material from said product container; and at leastone discharge jet positioned within said product container, said atleast one discharge jet connected to a source of fluid and beingpositioned so that said fluid discharged by said at least one dischargejet is directed towards said discharge opening to urge said materiallocated between said at least one discharge jet and said dischargeopening towards said discharge opening.
 19. The improved Wurster-typefluidized bed apparatus, according to claim 18, wherein said dischargeopening includes a displaceable cover, said cover being moveable betweena closed and open position.
 20. The improved Wurster-type fluidized bedapparatus, according to claim 18, wherein said fluid is pressurized gasand said material is at least one tablet.
 21. The improved Wurster-typefluidized bed apparatus, according to claim 18, wherein said fluid is acleaning liquid and said material is waste material located on saidorifice plate within said product container.
 22. The improvedWurster-type fluidized bed apparatus, according to claim 18, whereinsaid material is cleaning fluid located within said product containerduring a cleaning process.
 23. The improved Wurster-type fluidized bedapparatus for applying a coating liquid onto the surface of particles,said apparatus being of the type including a vertically disposedgenerally cylindrical product container having a peripheral wall, atleast one cylindrical partition defining a centrally located up-bedregion and a peripherally located down-bed region, said productcontainer further including an upper end connected to an expansionchamber, a lower end including an orifice plate having a plurality ofopenings for passage of fluidized air, a nozzle centrally locatedthrough said orifice plate and being adapted to generate a spray of saidcoating liquid upwardly into said up-bed, wherein said particles locatedwithin said product container circulate upwardly through said partitionand said spray of coating liquid, between said up-bed and said down-bed,said improvement comprising: a central first plenum located below saidorifice plate and being aligned with said up-bed and said partition,said first plenum being sized and shaped to direct gas upwardly throughsaid orifice plate and into said up-bed; a second plenum located belowsaid orifice plate positioned around said first plenum for directing gasupwardly through said orifice plate and into contact with particleslocated on said orifice plate; a first source of gas connected to saidfirst plenum; and a second source of gas connected to said secondplenum.
 24. The improved Wurster-type fluidized bed apparatus, accordingto claim 23, further comprising a flow control device positioned tocontrol the relative flow of gas through said first and second plenums.25. The improved Wurster-type fluidized bed apparatus, according toclaim 23, further comprising a flow measuring device positioned toindependently measure the flow of gas within said first and secondplenums.
 26. The improved Wurster-type fluidized bed apparatus,according to claim 23, further comprising a pressure measuring device incommunication with said first and second plenums, said pressuremeasuring device used to measure the pressure difference between saidfirst and second plenums.